Zinc oxide is well-known as a material used in UV blockers. For example, the prior-art references disclose applications for UV blockers in which zinc oxide is used (see Patent Documents 1 and 2). UV blockers are employed in cosmetics, and come into contact with human skin. Accordingly, it is desirable that the ingredients be chemically inert.
Although several reports have hitherto appeared on the synthesis of zinc oxide nanoparticles (see Patent Documents 3 to 5), no reports have appeared to date on zinc oxide microparticles which are spherical and have a particle diameter of about 50 to 1,000 nm and a good dispersibility in any liquid, such as water or an organic solvent, or on zinc oxide microparticle dispersions.
When a dispersion of zinc oxide particles is produced for the above applications, a stable dispersion cannot be obtained by the conventional method of simply dispersing fine particles of dried zinc oxide in a dispersion medium. This is because the zinc oxide particles aggregate, and it is necessary to break up these aggregations in order to obtain a stable dispersion.
Whether nanoparticles are synthesized by a gas phase process or a liquid phase process, they generally aggregate strongly unless aggregation is controlled following formation of the nanoparticles. Once the nanoparticles have strongly aggregated, it is generally difficult to break up the aggregations even by suitable treatment of the particles.
The prior-art reference discloses a technique for mechanically breaking up aggregations using ceramic beads (see Patent Document 6), but the problem in this case is the possibility of contamination by impurities. Moreover, a dispersant must be added to the solvent. For these reasons, zinc oxide microparticles which easily disperse (are resistant to aggregation) must be synthesized in such a way that aggregations do not have to be broken up by mechanical means or by dispersant addition.
Because nanoparticles are difficult to separate once they have aggregated, it should be possible to obtain easily dispersible zinc oxide particles by treating them to control aggregation before the nanoparticles aggregate; that is, as they are being produced.
If a dispersion medium in which a polymer has been dissolved is used as the locus of the reaction in this case, aggregation can be controlled as the zinc oxide microparticles are being produced, thereby presumably enabling a stable dispersion of zinc oxide microparticles to be obtained. Moreover, even if the zinc oxide microparticle dispersion is dried, it will probably be easy to re-disperse in a dispersion medium because it has been subjected to aggregation control treatment.
Instances where such a concept is used in a sol-gel process or a hydrolysis process have been reported (see Non-Patent Documents 1 to 4, Patent Document 7), although these reports do not relate to zinc oxide. No cases have been found in which such a concept has been applied to a refluxing process for precipitating zinc oxide microparticles.
Also, ultrafine particles of metal oxide and a production process for the same, and fine particles of metal oxide have been disclosed in respective prior-art documents (see Patent Documents 8 and 9). In these prior-art documents, the particles are spherical secondary particles having a particle diameter of about 50 to 1,000 nm and a small particle diameter distribution (standard deviation of particle diameter) which are formed by the aggregation of metal oxide primary particles having a particle diameter of about 10 to 20 nm. No mention whatsoever has been made of core-shell type zinc oxide microparticles having a good dispersibility in liquid or of core-shell type zinc oxide microparticle dispersions.    Patent Document 1: Japanese Patent Application Laid-open No. 2007-217380    Patent Document 2: Japanese Patent Application Laid-open No. 2007-197431    Patent Document 3: Japanese Patent Application Laid-open No. 2004-292282    Patent Document 4: Japanese Patent Application Laid-open No. 2004-182483    Patent Document 5: Japanese Patent Application Laid-open No. 2005-060145    Patent Document 6: Japanese Patent Application Laid-open No. 2004-35632    Patent Document 7: Japanese Patent Application Laid-open No. H2-92810    Patent Document 8: Japanese Patent Application Laid-open No. H6-218276    Patent Document 9: Japanese Patent Application Laid-open No. 2006-8629    Non-Patent Document 1: H. Yang, C. Huang, X. Su, Materials Letters, 60 (2006) 3714    Non-Patent Document 2: Z. T. Zhang, B. Zhao, L. M. Hu, J. Solid State Chem., 121 (1996) 105    Non-Patent Document 3: D. L. Tao, F. Wei, Mater. Lett., 58 (2004) 3226    Non-Patent Document 4: G. C. Xi, Y. Y. Peng, L. Q. Xu, M. Zhang, W. C. Yu, Y. T. Qian, Inorg. Chem. Commun., 7 (2004) 607
In light of these circumstances, the inventors conducted extensive research aimed at developing a process for producing nanosize zinc oxide particles which control nanoparticle aggregation and have long-term stability, and for producing dispersions of such particles. As a result, they have discovered that the use of a refluxing technique provides numerous advantages, including the ability to use an organic solvent and the absence of a need for a reaction initiator. They have also found that an inexpensive acetate may be suitably used as the starting material instead of an expensive alkoxide, thereby enabling the production of core-shell type zinc oxide microparticles which control nanoparticle aggregation, and of a dispersion of such microparticles. Further research by the inventors ultimately led to the present invention.